1. Field of the Invention
The present invention relates generally to radar systems, and particularly to local oscillator implementations in radar systems.
2. Technical Background
The term “radar” is an acronym for the phrase “radio detection and ranging.”A radar, therefore, transmits RF energy and listens for reflected return signals to detect a target and determine a target's location in space. The range is determined by what is commonly referred to in the art as the “radar equation.” When the radar is disposed on a moving platform, such as a ship or an aircraft, target location may be provided as a relative bearing or a true bearing. The relative bearing is the angle of the target relative to the ship's heading, whereas a true bearing is referenced from true north, i.e., it is the sum of the ship's heading and the target angle. If the target is airborne, the target altitude is obtained by multiplying the target range by the sine of the target's elevation angle. The target range is a function of the transmitted power, the received power, the antenna gain, the wavelength of the electromagnetic energy, the radar cross-section of the target and the time delay between the transmitted. An overview of a conventional radar system is provided in light of the basic concepts discussed above.
A simplified block diagram of a conventional radar system 10 is shown in FIG. 1. The conventional radar system 10 includes an antenna 12 coupled to circulator 14. The circulator 14 is coupled to a transmitter system 16 and a receiver system 18. A local oscillator block 20 provides the transmitter system 16 and the receiver system 18 with stable coherent local oscillator (LO) reference signals. The operation of radar 10 is controlled by a system control unit 24. One important function of system control 24 relates to the synchronization of the transmitter system 16 and receiver system 18 functions. This function requires stable coherent LO reference signals.
Transmitter system 16 includes a waveform generator 162 coupled to up-converter 160. Waveform generator 162 provides a pulse, or a series of pulses, to upconverter 160. Upconverter 160 modulates the pulses by multiplying them by an LO reference signal to thereby generate an RF pulse. Circulator 14 directs the RF signal to antenna 12. Antenna 12 radiates the RF signal into a predetermined coverage volume in accordance with the antenna design parameters. If there is a target disposed in the coverage volume, the radiated RF signal should be reflected by the target. A small portion of the reflected signal is captured by the antenna and directed into the receiver 18 via circulator 14.
The receiver 18 includes a downconverter 180 coupled to receiver front-end 182. Downconverter 180 amplifies and demodulates the incident reflected RF signal. Demodulation refers to the process of multiplying the received amplified signal by a LO reference signal to generate an analog signal characterized by a lower intermediate frequency (IF). The conversion from RF to IF is performed because IF signals are, in general, easier to process that are RF signals.
In any event, the IF signal is converted into digital data by receiver front end 182. Some of the more important receiver performance parameters include signal reception, signal-to-noise ratio (SNR), receiver bandwidth, and receiver sensitivity. Reception speaks to the receiver's ability to detect relatively weak signals. While the transmitter may provide several kilowatts of power, only a small fraction of that returns to the antenna in a reflected RF signal. Accordingly, the radar's ability to detect relatively weak signals determines the effective range of the radar. Every reflected return signal includes both the target's reflected signal and noise. The receiver sensitivity relates to the smallest return signal that the receiver is able to detect in a noisy environment. Sensitivity is usually specified in the milliwatt range.
The digital data may be in a single bit stream format or may employ complex signals that include in-phase and quadrature (I, Q) data signals. In any event, the receiver signal processor 184 correlates the received signal data with the transmitted signal to determine whether the data represents a legitimate target. The receiver signal processor 184 may perform many sophisticated calculations to distinguish a legitimate target echo from noise and background clutter. Doppler filtering may be employed to determine a target's velocity. Display 26 provides the processed data to the user in a user-recognizable format.
As those of ordinary skill in the art will appreciate, the Doppler effect relates to the apparent change in the frequency of a signal as the source of the signal moves relative to an observer. In everyday terms, the sounds made by a vehicle appear to change as it moves. The pitch increases as the vehicle moves toward us and decreases as the vehicle moves away. The Doppler effect applied to electromagnetic waves as well. A Doppler radar determines the radial velocity of a target by determining the frequency difference between the transmitted signal and the return signal. To put it simply, the frequency difference is proportional to the radial component of the target velocity. By measuring the Doppler effect, a Doppler radar is able to measure the velocity of a target as it moves in relation to the radar platform. Doppler radars are often used in military radar systems, weather radar, and in police radar guns. In fact, Doppler radar systems may be employed at the ballpark and be used to determine the speed of the pitcher's fastball.
A Doppler radar is a type of radar commonly referred to as a “coherent radar.” In a coherent system, the radar receiver determines target information using the phase of the reflected signal as well as the frequency and amplitude. Coherent radars compare the phase and/or frequency of a reflected signal to phase and/or frequency of a signal generated by a stable local oscillator source. Local oscillator 20 coherency is required to maintain phase relationships between return detections within multiple pulses within a dwell. Those of ordinary skill in the art will understand that the receiver uses the reflected energy from multiple pulses to facilitate target detection. If the system is not coherent, the reflections will not add properly. Further, the local oscillator 20 is often required to tune and re-tune to other frequencies to support calibration or other secondary receiver functions. If the timing signals generated by the oscillator in a subsequent receive period are not coherent with the previous timing signals generated by the oscillator, receiver signal processing related to Doppler filtering operations will not be performed properly. For example, when the receiver attempts to correlate the return signals with the transmitted waveform, spurious signals may be generated and clutter may not be canceled.
Referring to FIG. 2, a detailed block diagram of a conventional local oscillator 20 that may be employed in FIG. 1 is shown. System clock 22 (See FIG. 1) is configured in this example as an oscillator that provides an 8 MHz clock signal (FIG. 3A). FIGS. 3A-3D are timing diagrams of waveform outputs for the local oscillator shown in FIG. 2. Local oscillator 20 includes frequency synthesizer circuits (2000, 2002, 2004) disposed in series. FIG. 3B shows the 4 MHz output of synthesizer 200. FIG. 3C shows the 2 MHz output of synthesizer 202. Finally FIG. 3D shows the 1 MHz output of synthesizer 204. The local oscillator system 20 also includes several multipliers (2006-2014) that are used to mix the 3 MHz, 5 MHz, 6, MHz, 7 MHz, and 9 MHz clock signals used by the radar system 10. All told, local oscillator 20 provides unit frequency steps, i.e., 1-9 MHz signals in increments of 1 MHz. These different frequencies are used to tune the radar to different channels.
Because the oscillator and frequency synthesizers are always running, coherency is maintained over time. As noted above, local oscillator 20 is often required to switch between the various frequencies to support calibration or other secondary receiver functions.
Referring to FIG. 3A and FIG. 3B, if system control unit 24 orders LO 20 to switch between the 8 MHz signal 3000 to the 4 MHz signal 3002, and back again (See Pt. A), the phase of the 8 MHz clock 3000 is predictable because the clock has continued to run in synchronization with the 4 MHz clock signal 3002. If system control 24 selects the 2 MHz clock 3004 (FIG. 3C) and switches back to the 8 MHz clock 3000 at Pt. B, again, the phase of the 8 MHz clock 3000 is predictable and the receiver is able to perform coherent processing functions because there are no phase ambiguities when switching between frequency signals. FIG. 3D shows a 1 MHz clock 3006 which is shown to be in phase with the other clocks at Pt. C.
While the conventional local oscillator 20 shown in FIG. 2 provides coherent LO signals, the design has several drawbacks. The physical realization is relatively heavy, bulky, and power hungry. For example, the conventional local oscillator may occupy a volume that is greater than or equal to three cubic feet and dissipate over 200 W of power. Further, the circuit implementation requires additional components (not shown) for fine tuning resolution. This translates to higher cost. What is needed is a local oscillator that is lighter, more compact, less expensive, and more efficient from a power consumption standpoint than the conventional implementation.
Referring to FIG. 4, a block diagram of a portion of a conventional direct digital frequency synthesizer (DDS) 40 is shown. This is an implementation of a local oscillator that is lighter, more compact, less expensive, and more efficient from a power consumption standpoint than the conventional implementation shown in FIG. 2. DDS 40 includes a phase accumulator 400 that is typically coupled to a frequency accumulator (not shown). The frequency accumulator may be programmed to sequence through a predetermined modulation sequence, such as a chirp sequence, for example. The phase accumulator 400 is implemented as a digital counter. Each increment of the phase accumulator generates a signal corresponding to a timing waveform. In particular, the output of the phase accumulator 400 is a digital representation of the advancing phase of a clock signal having a predetermined frequency; the output is subsequently directed into phase-to-amplitude converter 402. The predetermined frequency is determined by the input to the phase accumulator, known as the frequency tune word which is provided by frequency control register 404. This frequency tune word determines how quickly the phase accumulator 400 advances through the clock cycle. The output of the phase-to-amplitude converter 402 is provided to a digital-to-analog converter (DAC). The diagram of FIG. 4 may be implemented in an integrated circuit, such as an FPGA or an ASIC. Therefore, the conventional DDS represents an improvement with respect to size, weight, and power consumption vis ávis the conventional oscillator depicted in FIG. 2.
However, as shown in FIG. 5, the DDS implementation has a major drawback in that it cannot provide multiple coherent frequency signals. Waveform 5000, for example, may represent the 8 MHz clock signal referred to above. At time (D), the frequency tune word register 404 is loaded to change the output frequency to a 6 MHz waveform 5002. FIG. 5 continues to show the 8 MHz signal as a dashed line for clarity of illustration. At time (E), the frequency tune word register 404 is again reloaded, this time to change the output frequency back to 8 MHz (See waveform 5004). Note that waveform 5004 is out of phase with respect to waveform 5000. As noted above, the phase ambiguity may prevent the receiver from properly performing Doppler processing, clutter cancellation and correlation filter functions.
What is needed, therefore, is a local oscillator implementation that provides stable coherent clock signaling in an implementation that is inexpensive, small in size and weight, and efficient from a power consumption standpoint. What is also needed is an oscillator that provides fine grain tuning resolution without the need for additional components.